1. Field of the Invention
The present invention relates to a magnetic memory device and a method of designing the same. More particularly, the present invention relates to a magnetic memory device which carries out a reading/writing operation of data using magnetic reversal in a magnetic substance element, and a method of designing the same.
2. Description of the Related Art
In recent years, the development of a magnetic random access memory (MRAM) using a magnetic resistance element as a memory element has carried forward. In the magnetic resistance element, a reading/writing operation is carried out using magnetic reversal which is brought about by a local magnetic field. The MRAM is expected to be a memory device of the next generation, because with MRAM, it is possible to operate at high speed.
The structure in which a tunnel insulating film is put between two magnetic substance films is called tunneling magnetroregistance (TMR). The TMR structure will be described below as an example of the magnetic resistance element. FIG. 1 shows an example of the TMR reported in 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 128). Referring to FIG. 1, an anti-ferromagnetic substance layer 30 formed of FeMn (10 nm), a ferromagnetic substance pin layer 31 formed of CoFe (2.4 nm), the tunnel insulating layer 32 formed of Al2O3, and a ferromagnetic substance free layer 33 formed of NiFe (5 nm) are stacked.
Conductive wiring lines are connected with the anti-ferromagnetic substance layer 30 and the free layer 33 to make it possible to apply a voltage. A magnetization direction of the pin layer 31 is fixed on a specific direction by the anti-ferromagnetic substance layer 30. The free layer 33 is formed to be easily magnetized and the magnetization direction of the free layer 33 can be changed by an externally applied magnetic field. It should be noted that a direction to which it is easy for the free layer to be magnetized is referred to as an easy axis and a direction to which it is difficult for the free layer to be magnetized and which is orthogonal to the easy direction is referred to as a hard axis.
In the TMR of the above structure, current flows through the tunnel insulating film 32 when a voltage is applied between the free layer 33 and the pin layer 31. In this case, a resistance value changes based on the relation of the magnetization of the free layer 33 and the magnetization of the pin layer 31. That is, when the magnetization directions are the same, the resistance is low. When the magnetization directions are different from each other, the resistance becomes high. Therefore, data can be stored by changing the magnetization direction of the free layer 33.
Next, an example using the above TMR as a memory element of a nonvolatile memory will be described with reference to FIG. 2. This example is reported in 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 130). Referring to FIG. 2, in this example, the TMRs 34 are arranged at the intersections of upper wiring lines B (B1, B2) 35 and lower wiring lines D (D1, D2, D3) 38 in an array manner. The upper wiring line 35 is connected with a free layer of the TMR 34. An anti-ferromagnetic substance layer of the TMR 34 is connected with the drain of a transistor 37 through a third wiring line 36. The gate of transistor 37 is controlled by read word line W (W1, W2, W3) 39. By flowing current through the two wiring lines B and D, a synthetic magnetic field is generated in the neighborhood of the intersection. The magnetization direction of the free layer is set based on the direction of the current. Thus, the resistance value of the TMR 34 can be changed.
FIG. 3 shows an example of the synthetic magnetic field condition when the magnetic reversal in this example is caused. Referring to FIG. 3, the following is described. That is, the magnetic reversal does not occur by only one magnetic field. There are condition areas (Write xe2x80x9c0xe2x80x9d, Write xe2x80x9c1xe2x80x9d) for the magnetic reversal using the synthetic magnetic field. Even if a writing operation is carried out to the TMR arranged at the intersection of the two wiring lines using this condition, the magnetic reversal does not occur because the magnetic field of one axis is applied only to the TMRs other than the TMR at the intersection. It should be noted that when a reading operation of the data from the TMR 34 is carried out, the transistor 37 connected with the TMR 34 is set to the ON state using a wiring line W to apply voltage to the TMR 34 through the wiring line B. The resistance value of the TMR is evaluated based on the flowing current.
However, in the memory device in which the conventional magnetic resistance elements are arranged in an array manner, the magnetization states of adjacent magnetic substance elements change by the magnetic field of the wiring line when the writing operation is carried out to the magnetic substance element. As a result, there is a possibility that the data cannot be read out, and the data is changed. This phenomenon will be described with reference to the figures.
Supposing that a weak magnetic field of a degree that the magnetization does not reverse is repeatedly applied to the magnetized magnetic substance in an opposite direction to the magnetization direction, the magnetization state is destroyed. Such a phenomenon is mentioned in xe2x80x9cMagnetic Engineering Lecture 5 Magnetic Thin Film Engineeringxe2x80x9d (Maruzen Co., Ltd., P.174). FIG. 4 shows a magnetic field condition when the magnetization state changes by applying a magnetic field to the easy axis and the hard axis of the magnetized magnetic substance. Also, FIG. 4 shows the dependence on the number of times when the magnetic field weaker than the magnetic field necessary for the magnetic reversal is repeatedly applied to the hard axis.
In this reference, a case is described that a sine wave magnetic field with the frequency of 1 kHz and the amplitude of 1.0 Oe is applied 105 cycles to the magnetic substance in the hard axis. In this case, the applied magnetic field is about 30 percent of the magnetic coercive force of 3.2 Oe of the magnetic substance, and the magnetization of the magnetic substance has reversed almost fully when the magnetic field 2.0 Oe is applied to the hard axis of the magnetic substance after the above repetitive application of the weak magnetic field. Also, in case that the magnetic fields are applied to both of the easy axis and the hard axis, the application of the magnetic field of 1.5 Oe to the hard axis causes the magnetization reversal when the magnetic field of 0.8 Oe is applied to the easy axis. Specifically, when the magnetic field of 2.0 Oe is applied to the easy axis using a bit line and the magnetic field of 2.0 Oe is applied to the hard axis using a word line, a synthetic magnetic field at the intersection of the bit line and the word line exceeds a critical magnetic field to cause the magnetic reversal. Thus, the magnetic reversal is not caused in an adjacent magnetic substance apart from the intersection and a writing operation can be carried out correctly. In this case, however, when the magnetic field of 1.0 Oe, which is a half of the writing magnetic field, is applied to the adjacent magnetic substance 105 times, the magnetic field of 2.0 Oe by the word line changes the magnetization state of the adjacent magnetic substance as mentioned above. In this way, when the number of cycles of the sine wave magnetic field exceeds a predetermined value, the magnetization state is changed with the magnetic field smaller than a static critical magnetic field. Consequently, the reliability of the magnetic memory device reduces remarkably.
In the magnetic memory device at present, the above phenomenon is not a problem because the interval between the word lines or the bit lines is wide so that the magnetic field which reaches an adjacent cell is small. However, when the magnetic memory device would be integrated in a high density in the future so that the interval between the elements is made narrow, it would be not possible to ignore the magnetic field from the adjacent cell.
Also, with the above problem, there is not an appropriate index to set the influence of the magnetic field to the adjacent cell to a desired state. Therefore, the pitch of the magnetic substance elements and the interval between the wiring line and the magnetic substance element cannot be easily set, on the design of the magnetic memory device.
In conjunction with the above description, a high density non-volatile ferromagnetic random access memory is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 9-509775). In this reference, a non-volatile ferromagnetic random access memory element is comprised of first and second ferromagnetic layers. At least one of the first and second ferromagnetic layers has a magnetic moment on a same plane. A non-magnetic metal layer is inserted between the first and second ferromagnetic layers. A first end non-magnetic conductive layer is provided for an end of the ferromagnetic random access memory element. A second end non-magnetic conductive layer is provided for an opposite end to the above end in the ferromagnetic random access memory element. A second end conductive layer is provided to be perpendicular to the magnetic moment of at least one of the first and second ferromagnetic layers and to limit a conductive path through which current flows from the first magnetic layer to the second magnetic layer via the non-magnetic metal layers.
Also, a magnetic memory device is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 10-177783). In the magnetic memory device of this reference, the difference between two or more ferromagnetic substance layers stacked via non-magnetic layers in the direction of magnetization is stored as data. The difference in the direction of the magnetization is read out as a resistance value. A first data storage section is provided on a substrate and a second data storage section is provided adjacent to the first data storage section. A magnetic field forming electrode is provided adjacent to the first and second data storage sections to apply the magnetic fields of opposite directions to the first and second data storage sections at the same time. The magnetic field forming electrode is provided on the first data storage section through a first insulating film, and the second data storage section is provided on the magnetic field forming electrode through a second insulating film.
Also, a magnetic memory device is disclosed in Japanese Laid Open Patent Application (JP-P2000-106462A). In this reference, the magnetic memory has a ferromagnetic tunnel junction structure of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer or ferromagnetic double tunnel junction structure of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer/tunnel barrier layer/ferromagnetic layer. The tunnel resistance of the magnetic memory device changes, depending on a relative angle between magnetization directions of the ferromagnetic layers. The magnetic memory device has a magnetic resistance ratio which increases with a positive inclination in accordance with the increase of a bias voltage. Also, the decrease of the magnetic resistance ratio which accompanies the rise of the temperature is small. In the tunnel barrier layer, the value of S/("PHgr")xc2xd meets 10xe2x89xa6S/("PHgr")xc2xd, when tunnel barrier height is "PHgr" [eV], and tunnel barrier width is S [angstroms].
Also, a quantum random address memory is disclosed in Japanese Laid Open Patent Application (JP-P2000-285669A). In this reference, the quantum random address memory is comprised of a plurality of address ports which give a plurality of addresses in a low dimension, a data output section, a plurality of magnetic nano-memory elements, and a plurality of non-linear mixer elements arranged quasi-randomly to combine a one side portion of the plurality of address ports and the data output section to the plurality of magnetic nano-memory elements in a high dimension. An other side portion of the plurality of address ports and the data output section are combined with the plurality of magnetic nano-memory elements , and the plurality of magnetic nano-memory elements are larger than the plurality of addresses in the low dimension.
Therefore, an object of the present invention is to provide a magnetic memory device and a method of designing the magnetic memory device, in which an index to evaluate the influence of a magnetic field to a neighbor cell can be given.
Another object of the present invention is to provide a magnetic memory device and a method of designing the magnetic memory device, in which the memory characteristic can be kept even if magnetic substance elements are arranged in a high density.
In an aspect of the present invention, a magnetic memory device includes a plurality of word lines extending into a first direction, a plurality of bit lines extending into a second direction to intersect the plurality of word lines, and a plurality of magnetic memory cells which are provided at intersections of the plurality of word lines and the plurality of bit lines, and each of which includes a ferromagnetic film. Data is written in a first one of the plurality of magnetic memory cells with a synthetic magnetic field generated by first current flowing on a specific one of the plurality of word lines associated with the first magnetic memory cell and second current flowing on a specific one of the plurality of bit lines associated with the first magnetic memory cell. At this time, a distance d between the magnetic memory cell and one of the specific word line or the specific bit line meets the following equation (1),
dxe2x89xa6pxc3x97(xcex3/(1xe2x88x92xcex3))xc2xd(0 less than xcex3 less than 1)xe2x80x83xe2x80x83(1)
where p is a distance between the first magnetic memory cell and a second magnetic memory cell adjacent to the first magnetic memory cell in a specific one of the first direction and the second direction, and xcex3 is a strength ratio of a component of the synthetic magnetic field in the specific direction in the second magnetic memory cell.
Here, it is desirable that an easy axis of the first magnetic memory cell is in the first direction.
Also, the distance d is desirably defined based on a center position of the ferromagnetic film and a center of a current distribution in one of the specific wiring line and the specific bit line.
Also, it is desirable that the distance p between the first magnetic memory cell and the second magnetic memory cell is equal to or less than 1 xcexcm.
Also, it is desirable that the distance p between the first magnetic memory cell and the second magnetic memory cell is equal to or less than 1 xcexcm, and the xcex3 is equal to or less than 0.3.
In another aspect of the present invention, a magnetic memory device includes a plurality of write word lines extending into a first direction, a plurality of read word lines extending into the first direction in parallel to the plurality of write word lines, a plurality of bit lines extending into a second direction to intersect the plurality of write word lines, and a plurality of magnetic memory cells which are provided at intersections of the plurality of word lines and the plurality of bit lines, and each of which includes a ferromagnetic film and a conductive drawing line connected with a corresponding one of the plurality of read word lines. Data are written in accessed magnetic memory cells as first ones of the plurality of magnetic memory cells with synthetic magnetic fields, each of which is generated by first current flowing on one of the plurality of write word lines associated with a corresponding one of the accessed magnetic memory cells and second current flowing on one of the plurality of bit lines associated with the corresponding accessed magnetic memory cell. At this time, a distance d between an end-side one of the accessed magnetic memory cells and one of the associated write word line or the associated bit line meets the following equation (2),
dxe2x89xa6pxc3x97(xcex3/(1xe2x88x92xcex3))xc2xd(0 less than xcex3 less than 1)xe2x80x83xe2x80x83(2)
where p is a distance between the end-side accessed magnetic memory cell and a non-accessed one of the plurality of magnetic memory cells adjacent to the end-side accessed magnetic memory cell in a specific one of the first direction and the second direction, and xcex3 is a strength ratio of a component of the synthetic magnetic field in the specific direction in the non-accessed magnetic memory cell.
Here, it is desirable that an easy axis of the accessed magnetic memory cell is in the first direction.
Also, the distance d is desirably defined based on a center position of the ferromagnetic film of the end-side magnetic memory cell and a center of a current distribution in one of the write wiring line associated with the end-side magnetic memory cell and the bit line associated with the end-side magnetic memory cell.
Also, the distance p between the end-side accessed magnetic memory cell and the non-accessed magnetic memory cell may be equal to or less than 1 xcexcm.
Also, the distance p between the end-side accessed magnetic memory cell and the non-accessed magnetic memory cell may be equal to or less than 1 xcexcm, and the xcex3 may be equal to or less than 0.3.
Also, a first wiring line pitch between the bit lines corresponding to the accessed magnetic memory cells may be different from a second wiring line pitch between the end-side accessed magnetic memory cell and the non-accessed magnetic memory cell. In this case, the first wiring line pitch is desirably smaller than the second wiring line pitch.